[Deeper understanding of fetal acid-base equilibrium].

Abstract

Problem: Actual pH-measurements, which are used worldwide for quality control in obstetrics, give no information about the extent of metabolic and respiratory pathways involved in the synthesis of H(+)-ions. Therefore we looked for a new method to quantify the de novo-synthesis of H(+)-ions due to both pathways and to correlate them with the clinical condition of the newborn.

Methods: Using pH-, pCO2 (mmHg)- (and pO2 (mmHg)) measurements in umbilical blood (artery (UA), vein (UV)) of 8882 newborns, which were delivered vaginally in vertex position without cord-entanglements and without major malformations, the pHqu40 (metabolic pathway) and the pHnm (non-metabolic pathway) were computed. Thus both components of fetal acidosis could be determined together with actual pH in one unity i.e. H(+)-ion-concentration (nmol/l). By introduction of a reference-point, which denotes the mean actual pH- (7.373) and the mean pCO2- (36.4 mmHg) value in umbilical venous blood of 599 Apgar 10-babies it is possible to compute the mean de novo-synthesis of H(+)-ions in different clinical neonatal conditions (Apgar-score 1 min).

Results: In the case of slightly depressed neonates (Apgar 7-10) the respiratory acidosis dominates the metabolic one. In moderately asphyxiated newborns (Apgar 4-6) both components are equally involved. In severe fetal asphyxia (Apgar 0-3) both components can probably be determined by extrapolation: the metabolic component seems to be dominant. The computed mean actual pH-value in babies scoring Apgar 0 amounts to approximately 7.100 +/- 0.075.

Conclusions: The diagnostic range of Apgar-scoring and pH-measurements is not congruent: newborns with actual pH-values in UA-blood between 6,7 and approximately 7,1 do show Apgar-scores (1 min) of 0 or 1 without a possibility to further differentiate the degree of asphyxia by clinical criteria. This means, that actual pH-measurements are of high clinical importance in severely asphyxiated newborns. They should not be abandoned. The intercorrelation of some variables of the fetal acid-base-balance is demonstrated in 738 newborns sharing Apgar-scores of 8 after one minute. The diagnostic potential of acid-base-variables in UV-blood as a mirror of placental function is outlined.